As a general rule, a noise-reducing device for an air-conditioning circuit of a motor vehicle is formed of an acoustic capacity that is mounted in line in a duct of this circuit between a compressor and an evaporator, so as to attenuate the transmission of the sound waves generated primarily by the compressor and propagating in the cooling fluid inside the conduit, via a sudden enlargement of the cross section of the passage for the fluid entering this capacity. This enlargement of the flow cross section at the entry into the capacity results in a known manner in the creation of a sound wave with opposite phase adding to the disturbing wave to generate an attenuated wave of lower amplitude.
Such an acoustic capacity is usually situated at a location of the duct of the air-conditioning circuit that is relatively far from the compressor, essentially because of the volume that this capacity occupies and the room that is often lacking in the driving engine assembly of the motor vehicle comprising this compressor to install the capacity in the immediate vicinity of the latter and, in situations where such installation of the capacity in the immediate vicinity of the compressor would nevertheless be feasible, because of the undesirable mechanical vibrations that are generated by the presence of the additional mass formed by this capacity and which may cause the corresponding duct to rupture.
This relatively great distance of the acoustic capacity relative to the compressor notably has the effect of generating an area of resonance between these two components in which noise may be self-amplifying. As a result of this the attenuation of the noise in the duct of the air-conditioning circuit is still open to improvement.
These acoustic capacities thus enable some degree of attenuation of the noise generated by the compressor and propagating in the duct of the air-conditioning circuit. On the other hand, they simultaneously have the drawback of generating significant head losses for the fluid flowing through them because of the sudden modification of the flow cross section that characterizes them. Furthermore, they have a relatively high cost of manufacture and of installation on the duct of the air-conditioning circuit.
The document EP-A1-1 864 838 in the name of the Applicant describes in this context a cooling fluid conduit for such an air-conditioning circuit which makes it possible to remedy this drawback at the same time as achieving satisfactory acoustic attenuation in this circuit. This conduit is provided in its internal space with a noise-reducing device the radially external face of which espouses the wall of the conduit and which has entry and exit ends forming at least one axial flow channel of the fluid in the device.
The document EP-A1-2 106 943 also in the name of the Applicant describes a cooling fluid duct for such an air-conditioning circuit including a male end fitting that is intended to be connected in fluid-tight manner to a female end fitting and inside which there is mounted a globally tubular noise-reducing device defining between two ends of this device an axial flow channel for the fluid and with the male end fitting at least one annular dead volume that does not communicate with this channel at one of these two ends at least. This noise-reducing device also achieves satisfactory attenuation in this circuit.